1. Field of the Invention
The present invention relates to a magnetic recording and reproducing apparatus of a type that uses a magnetic tape and a rotary head, such as video tape recorders, and more particularly to a means for controlling the tape tension and tracking as well as the amount of spacing between the magnetic head and the magnetic tape to permit high-density recording and high-speed replay.
2. Description of the Background Art
FIG. 48 shows the outline construction of a general tape driving system in a rotary head type magnetic recording and reproducing apparatus, such as video tape recorder (VTR) cited in "Jiki Kiroku Gijutsu Nyumon (Introduction of Magnetic Recording Technology)" (by Yokoyama, published by Sogo Denshi Shuppansha, page 187).
The path in this tape drive system through which the magnetic tape T travels consists of the following components:
a supply reel 15; a back tension post 36 to detect the tension of the magnetic tape T running through the path; a full-width erase head 37 to erase the information recorded by the rotary head on at least a part of the magnetic tape T; an impedance roller 55 and a guide roller 17 to stabilize the movement of the magnetic tape T; an inclination post 18 to give a certain inclination to the magnetic tape T fed onto the circumference of the head cylinder D made up of an upper cylinder D.sub.1 and a lower cylinder D.sub.2 ; a circumferential edge of the head cylinder D having a rotary head (not seen in this figure because of the magnetic tape T obstructing the sight); an inclination post 20 to relieve the magnetic tape T leaving the head cylinder D of the inclination; a guide roller 21; an impedance roller 70; an erase head 73 to erase information including voice and control information recorded on the linear track formed parallel to the edge of the magnetic tape T; a record and reproduce head 74 for recording and reproducing information such as voice information and control signal to and from the linear track; a pinch roller 75 to hold the magnetic tape T with a constant pressure between it and a capstan 22, which is driven by a capstan motor not shown to drive the magnetic tape T at a specified speed; a guide roller 80; and a takeup reel 23.
In the above construction, the magnetic tape T is gripped by the pinch roller 75 and the capstan 22 rotating at a specified speed and is pulled by the rotating force of the capstan 22 so that it is driven from the supply reel 15 through the back tension post 36 to the guide roller 80 and finally wound on the takeup reel 23. In a part of this tape passage, the magnetic tape T is wound helically on the head cylinder D at a specified angle to allow the rotary head that rotates with the upper cylinder D.sub.1 to magnetically record or reproduce information to/from the helical track on the tape.
In this part of the tape passage, the recording and reproducing or erasing is also carried out by the full-width erase head 37, erase head 73 and record and reproduce head 74 in addition to the rotary head. That is, when all the information to be recorded on the magnetic tape, such as the video signal, audio signal and control signal, is new, the information present in the full width of the magnetic tape T is erased by the full-width erase head 37 and then the video, audio and control signals are recorded by the rotary head and the record and reproduce head 74. During the after-recording of voice or during the rewriting of only the control signal, only the linear track area of the magnetic tape is erased by the erase head 73 and then new voice and control information is recorded in the linear track by the record and reproduce head 74.
The record format used in recording such video, audio and control signals in helical track or linear track can be made compatible with video signal recording schemes generally known as VHS and .beta. systems and a digital voice signal recording scheme generally called DAT.
In this kind of magnetic recording and reproducing apparatus, high density recording and reproducing requires keeping the spacing between the magnetic head and the tape at a constant value. Further, to perform special reproduction such as high-speed replay and still replay without generating noise requires an additional control of making the magnetic head follow the recorded helical track.
That is, to record, reproduce or erase the magnetic tape with good characteristic in a high frequency range in the magnetic recording and reproducing apparatus, it is necessary to keep spacing between the magnetic head gap portion and the magnetic tape at a small constant value. If this requirement is not met, a desired characteristic cannot be obtained. This is a known fact.
In the magnetic recording and reproducing apparatus shown in FIG. 48, when the tension of the magnetic tape T between a tape portion held by the impedance roller 55 and the guide roller 17 and another tape portion held by the capstan 22 and the pinch roller 75 is increased, the air gap or spacing between the magnetic tape T and the rotary head, erase head and record and reproduce head becomes smaller, improving the high-frequency characteristic. But when the tension is reduced, the gap between the magnetic tape T and these magnetic heads increases, deteriorating the high-frequency characteristic.
However, when the spacing becomes so small that the magnetic tape T directly contacts the magnetic heads, the wear of the magnetic heads increases deteriorating their working life or damaging the tape. Too small a spacing also poses a problem of degrading the magnetic tape during the still replay because the still replay reproduces the same track continuously.
It has therefore been required that the spacing between the magnetic tape and the magnetic head be kept constant. As one means of meeting this requirement, there is a known method for controlling the back tension of the magnetic tape.
FIG. 49 shows an example of the back tension controller applied to the magnetic recording and reproducing apparatus of FIG. 48. This back tension controller controls the back tension of the tape to keep the spacing constant. The force required to draw out the magnetic tape T from the supply reel 15 mounted on the supply reel mount 15a, i.e., the back tension, is controlled to keep the tension of the magnetic tape T at a specified constant value. The supply reel 15 and the back tension post 36 are also shown in FIG. 48.
A band brake 76 with one end fixed is provided to the circumference of the supply reel mount 15a to restrict the rotation of the reel mount. The other end is engaged with a pin 77b on a tension control arm 77 that is pivotable about a shaft 77a.
The other end of the tension control arm 77 opposite to the shaft 77a is fitted with the back tension post 36 with which the magnetic tape T engages. The intermediate portion of the tension control arm 77 is formed with a connector portion 77c, to which a tension spring 78 is attached at one end, with the other end connected to the tip of a tension adjust lever 79.
The tension adjust lever 79 can be rotated about its shaft 79a. By rotating the lever 79 to change the pulling force of the tension control arm 77 through the tension spring 78, it is possible to adjust the reference tension of the tension controller.
In the tension controller of FIG. 49, as the tension of the magnetic tape T increases, the tension control arm 77 rotates clockwise against the pulling force of the tension spring 78. The pin 77b--to which the free end of the band brake 76 mounted around the circumference of the supply reel mount 15a is attached--moves toward the right in the figure loosening the band brake 76, with the result that the force restricting the rotation of the supply reel mount 15a becomes smaller, increasing the feed of the magnetic tape T and reducing its tension.
Conversely, when the tension of the magnetic tape T decreases, the tension control arm 77 is rotated counterclockwise by the force of the tension spring 78. The pin 77b--to which the free end of the band brake 76 mounted around the circumference of the supply reel mount 15a is attached--moves toward the left in the figure increasing the braking force of the band brake 76, with the result that the feed of the magnetic tape T is reduced and the tension of the tape is increased.
The basic or reference tension can be adjusted by rotating the adjust lever 79, as mentioned above.
In the conventional magnetic recording and reproducing apparatus, the tension of the magnetic tape is adjusted by the mechanism described above to absorb moderate variations of the tape tension at the tape supply position, thereby holding the spacing between the magnetic tape and the head constant.
Such a conventional system does not relate the spacing between the magnetic tape and the head to the tape tension in the control of the spacing. The control of the spacing at a constant value for securing the steady head contact is achieved by making the surface accuracy of the magnetic tape uniform and by performing a strict quality control during manufacturing process on the head cylinder's surface smoothness and the window shape as well as on the shape and the amount of projection of the magnetic head.
However, it is very difficult to control the spacing with a precision required for high density recording on the magnetic tape. In apparatuses that perform special replay in which the replay and search is made while feeding the magnetic tape at high speed, the variations in the tape speed increase producing temporary tensions in the tape. This makes it practically impossible to keep the tape tension and spacing at constant values by a mechanical control system.
In other words, the control of the spacing making use of such a mechanical system may be applicable to systems that require only a low-frequency control range or not-so-high recording density. The above control method poses a problem when the line recording density is increased.
As the magnetic tape in recent years has increasingly fine surface smoothness as with the vapor deposition tape, the spacing accuracy or the precision required of the contact head also increases, raising the requirement of the control accuracy and making it increasingly difficult to deal with this problem by the mechanical control of the head contact.
During the special replay such as high-speed search, tape speed variations become large, causing temporary tension in the tape. In such a case, it is difficult to keep the tape tension and therefore the spacing at constant values by a mechanical control system. Hence the head contact inevitably deteriorates.
Next, we will explain the tracking process in which the magnetic head traces the helical track.
In the normal speed (1-time speed) replay, the magnetic head tracing on the magnetic tape is done by keeping the tape speed and the rotating speed of the rotary head equal to those during the recording, so that the angle between the magnetic head trace and the edge of the magnetic tape becomes equal to the angle of the track being recorded.
However, if the replay is performed by changing only the magnetic tape speed from that during the recording, with the rotating speed of the rotary head set equal to that during the recording, the angle in which the magnetic head traces does not match the angle of the recording track, causing deviations from the track (or simply referred to as an inclination error) and therefore noise on the reproduced picture.
In the helical scan type VTR mentioned above, the conventional auto-tracking reproducing apparatus which matches the locus of the magnetic head with that of the recording track normally has a video signal reproducing magnetic head mounted on an electro-mechanical conversion element (referred to as a head actuator). During replay, the head actuator drives the magnetic head in a direction perpendicular to or in a direction having a vertical component with respect to the movement of the recording track to make the magnetic head automatically trace the recording track.
Various kinds of auto-tracking control techniques for automatically tracing the recording track by the magnetic head mounted on the head actuator have been proposed and already put into practical use.
As to the detection of tracking error, as is known with the 8 mm VTR format, several kinds (four example, four) of low-frequency tracking pilot signals outside the video signal frequency band are superimposed on the video signal and recorded over several tracks (four tracks) so that different pilot signals are adjacent to each other. With this pilot signal method, a tracking error signal may be detected according to the difference in cross-talk level between the left and right tracks during the replay.
As to the wobbling method employed in the 1-inch VTR of Anpex Corp. and the D-2 format digital VTR DVR-10 of Sony Corp., the magnetic head is forcibly subjected to fine vibrations at a constant frequency or so-called wobbling frequency in the track width direction. The reproduction envelope signal from the magnetic head at this time is synchronously detected at the wobbling frequency to produce a tracking error signal.
As to the so-called mountain climbing method employed in NV-10000 VHS-VTR of Matsushita and HV series and F75 VHS-VTR of Mitsubishi Electric, the reproduction envelope signal from the magnetic head is sample-held at the center of the field read out. Then, the voltage applied to the actuator or the rotating phase of the capstan motor is changed one step(for example, increased one step), and the envelope level of the frame is compared with the sample-held value. This series of operations is continued until the next frame envelope level is smaller.
Then, if the next frame envelope becomes smaller, the direction of applied voltage is reversed and the similar operation is continued to converge the reproduction envelope toward the maximum value.
The conventional auto-tracking reproduction apparatus utilizes the above-mentioned various tracking error detection methods whereby tracking errors are detected and fed back to the head actuator incorporated in the head cylinder.
Such movable heads are generally used not only for the DTF control to correct the tracking error at times of normal speed replay but also frequently used for special replays such as high-speed replay, slow replay and still replay.
FIG. 50 shows the outline of a system, published in the National Technical Report Vol.28, No.3 (June, 1982), page 41, as one example case where such a movable head is applied for noiseless special replay.
The process of high-speed special replay according to the conventional technique is briefly explained below. In the block diagram of FIG. 51, the rotating magnetic head H is driven by the head actuator 14 in a direction perpendicular to the direction of the tape travel. The reproduction envelope signal from the magnetic head H is fed to the tracking error detector 16, which detects the amount of tracking error.
The inclination correction pattern generator 18 uses the tape speed information from the frequency generator representing the rotating speed of the capstan to generate an inclination correction pattern that causes the magnetic head H to trace the track in such a way that the angle at which the magnetic head H scans over the tape is equal to the angle of the recording track.
The tracking error signal from the tracking error detector 16 and the inclination correction pattern from the inclination correction pattern generator 18 are added up by an adder 410 to produce a head actuator drive signal that causes the magnetic head to move in the direction of the magnetic tape travel.
As described in more detail, during the normal speed (1-time speed) replay, the magnetic head H traces on the tape at an angle equal to the angle of the recording track. However, during the replay at different speeds the angle at which the magnetic head H traces over the tape is not equal to the angle of the recording track, resulting in a track shift (referred to as an inclination error) and generating noise on the reproduced picture.
FIGS. 52 and 53 show the relationship between the recording track pattern on the magnetic tape and the locus of the magnetic head. The rotating speed of the rotary head remains the same as that during the recording. The tape is driven in the forward direction in FIG. 52 and in the backward direction in FIG. 53, at a 5-times replay speed, which means the tape is driven and replayed at 5 times the recording speed.
Reference numeral A in FIG. 52 shows the locus of the magnetic head at times of recording and normal replay. When the magnetic tape is driven at 5 times the normal speed in the forward direction, the angle of the magnetic head locus becomes large as shown at B so that the magnetic head crosses five tracks (1)-(5) in the same period that it crosses the magnetic tape widthwise.
FIG. 53 shows the locus of the magnetic head when the magnetic tape is driven at 5 times the normal replay speed in a direction reverse to the locus of the magnetic head that was described during the normal replay. It describes a locus C that intersects the tracks -(1) to -(5) at a small angle.
To realize noiseless replay of the reproduced picture also during the special replay mode in which the magnetic tape is driven at speeds different from that for recording, as in the 5-times speed replay, the loci B, C must be corrected so that they trace the recorded tracks.
FIG. 54 shows the outline of inclination error patterns produced in a so-called guard bandless recording scheme VTR which separates signals of adjacent tracks from each other that have no space therebetween. These inclination error patterns are produced during an n-times speed replay (n is an arbitrary real number) by a reproducing apparatus that uses a rotary head having two magnetic heads arranged at opposite positions 180 degrees apart.
Let T stand for one-half period of the rotary head rotation and tp for track pitch. Then, the inclination error during the n-times speed replay is given by tp(n-1). The value of the inclination error is expressed as a function that has a replay speed ratio n as a parameter. In other words, the inclination error changes according to the tape travel speed.
The inclination correction pattern generator 18 in FIG. 51 may use, for example, an FG signal from the capstan frequency generator as the tape speed information to generate the inclination correction pattern.
When the inclination correction pattern is fed to the head actuator 14 of FIG. 51, the locus of the magnetic head H is corrected in its inclination so that the magnetic head moves parallel to the recorded track even during replay at other than the normal replay speed.
However, simply displacing the magnetic head H according to the angle of the recorded track cannot avoid deviation of the head from the track because of the difference in linearity between the recording track and the magnetic head H or because of the phase shift of the track. For the purpose of preventing this track shift, normally a closed-loop auto-tracking control system, shown enclosed by dotted line in FIG. 51, is added.
The control methods for the auto-tracking control system include the pilot scheme, wobbling scheme and mountain climbing scheme. To produce a quality picture even during replay at other than normal replay speed, it is necessary to make the magnetic head follow the non-linearity of the recorded track (track bend). It is therefore preferred that the pilot scheme and wobbling scheme that can offer relatively wide control frequency range be adopted. The control method and operation of the auto-tracking control system are known and hence their explanation is omitted.
In digital VTRs that digitally record and reproduce Hi-Vision signal, video signal and audio signal, the amount of information on the signals to be recorded is very large. To permit long hour recording on a limited-size cassette tape, a high-density recording and high-precision DTF control are essential for the reproducing technique.
The DTF device in conventional VTRs has only a movable head with the tracking error correction means mounted on the head cylinder, so that the DTF control performance is determined by the performance of the head actuator that drives the movable head.
A desirable head actuator for the DTF control having high precision and wide frequency range should have no phase shift in a relatively high frequency range of, for example, between 1 kHz and several kHz. To suppress phase shift in a frequency range up to high frequency, a mechanical characteristic that does not resonate up to high frequency is necessary.
The first-order resonance frequency of the actuator mechanical characteristic is given by taking a square root of an actuator spring constant divided by the movable portion mass and dividing the square root by 2.pi.. So, to increase the first-order resonance frequency, it is common practice to reduce the movable portion mass or increase the actuator spring constant.
As mentioned earlier, the movable head is generally used not only for the DTF control during the normal speed replay but also for special replays. The high-speed noiseless reproducing apparatus in the conventional VTR corrects the tracking error by moving the magnetic head in the widthwise direction of the recorded track by the head actuator. Hence, the amount of tracking error that can be corrected is limited to the movable range of the head actuator.
In the conventional construction, therefore, the head actuator must be incorporated into the head cylinder whose outer diameter is determined by industrial standard. This requires the size of the head actuator to be reduced. Further, since the head actuator is required to have high spring stiffness suited for high resonance frequency essential for wide band DTF, the movable range of the head actuator is necessarily limited, lowering the performance of the high-speed special replay.
As mentioned above, the conventional apparatus has the problem that it is physically impossible to realize both the high-precision wide-band DTF control having a control band of several hundred Hz and the noiseless high-speed reproducing performance at as high as several tens of times the normal replay speed.
Since the tape tension control mechanism in the conventional magnetic recording and reproducing apparatus is constructed as described above, no special tape tension control is performed during high-speed tape feeding except that a constant load is applied to the tape in a direction opposite to the tape feed direction. Thus, the tape tension control mechanism cannot correctly respond to the temporary tension variations, causing damage to the tape. Another drawback of this mechanism is that changes in the contact state between the magnetic head and the tape due to the tension variations result in output variations, easily degrading the information.
Further, the conventional tension control mechanism has a narrow tension control frequency band and thus can suppress tension variations only at several Hz or lower. Therefore, with the VTRs that perform high-density recording and reproducing like digital VTRs, it is impossible to keep the spacing between the magnetic head and the magnetic tape at an optimum value at all times, rendering the good recording and reproduction difficult.
While the conventional magnetic recording and reproducing apparatus with the above construction can hold the tape tension and secure the head contact in the low frequency band on the tape supply side, it cannot remove variations in the tape tension at the head cylinder caused by load variations in the tape drive system. The variations in the spacing between the magnetic tape and the head resulting from tape tension variations at the cylinder head hinders the high-density recording and causes damage to tape and head wear.
The spacing variations are also produced by the deviation of the head cylinder, which compounds the problem. In the magnetic disk apparatuses, too, spacing variations resulting from the plane vibrations of the disk pose a serious problem. In the conventional magnetic recording and reproducing apparatuses, therefore, the important task to be addressed is to keep the spacing constant, enable high-density recording and at the same time improve reliability of the apparatus and recording mediums.